The following publications are believed to represent the current state of the art: U.S. Pat. Nos. 2,037,626; 2,135,119; 2,297,951; 2,201,155; 3,549,964; 3,672,610; 3,779,488; 3,809,341; 4,875,644; 4,399,967; 4,458,865; 4,501,398; 5,206,806 and 7,084,553.